This invention relates to dental burs and, more particularly, to dental burs of the type in which the cutting head is formed from carbide blanks which are fixedly connected to one end of stems or shanks formed from conventional steel for attachment to the chuck of a dental handpiece. The present invention more specifically pertains to the type of dental burs in which the burs are provided with a plurality of cutting teeth that extend longitudinally of the cutting head, each tooth being provided with a cutting edge spaced circumferentially even distances from adjacent cutting edges, said cutting edges being formed by grinding operations. When forming the cutting heads from carbide, diamond grinding wheels are conventionally used to form the edges by grinding operations.
The invention also pertains to dental burs in which the cutting edges are of the helical type. Dental burs of the type to which the present invention pertains are provided with a chip-receiving gash or groove adjacent one edge of each tooth and the opposite edge of the tooth is formed by a so-called relief surface which provides clearance immediately to the rear of the cutting edge of each tooth and thus, offer no resistance relative to the sidewall of a cavity, for example, which is being prepared in a tooth by the use of said bur.
One of the objectives of burs of this type which presently are in use is to provide a chip-receiving gash or groove having a depth adequate to accommodate chips removed from a tooth in which the bur is operating but, in order to provide a gash or groove of substantial depth, it is necessary at present to sacrifice some of the strength of the cutting edge which is afforded by the relief surface that backs up the cutting edge when rotating in operative direction. This is due to the fact that, in order to grind a relatively deep chip-receiving gash or groove, it is necessary to form a relatively steep relief surface which results in a sharper cutting edge than otherwise and, particularly when the bur is formed from carbide, which is quite brittle, the sharper cutting edge is more readily subject to chipping when encountering hard substances than if the cutting edge were less sharp and a larger angle extended between the opposite faces of the cutting edge. The foregoing phenomenon is particularly prevelant when carbide burs having helical teeth thereon are ground by means of formed or shaped grinding wheels operating, for example, about a fixed axis while the bur is rotated about its axis when being fed forwardly against the periphery of the grinding wheel during such axial rotation of the bur to provide the helical outline of the cutting teeth by simultaneously forming the cutting face and relief surface with a single cutter during a single pass along the blank to form each tooth. Under such circumstances, the grinding of cutting teeth on carbide burs has, historically, been achieved by the use of profile diamond grinding wheels of commercial type.
Also in regard to the grinding of carbide burs of the type referred to by the use of profile grinding wheels, especially in regard to making burs having helical teeth extending generally in axial direction of the bur, a certain amount of generation of the curved surface which forms both sides of each tooth is necessary and this generally results in the formation of a cutting face having a negative cutting rake on said surface as distinguished from a positive cutting rake which is more efficient than a negative rake, as well as a relief surface which has a lesser angle between the clearance surface and a plane perpendicular to the cutting face at the cutting edge of each tooth.
The foregoing deficiencies and undesirable characteristics of conventional carbide burs, which are manufactured at present by conventional techniques and are commonplace in the trade, are obviated by the present invention in which superior cutting teeth on a carbide type bur are provided having improved structural and operational characteristics which tapered fissure burs and on inverted cone burs, details of which are set forth in detail hereinafter.